Illinois coal forests home to unique arachnid with spiny legs — study

A 308-million-year-old arachnid called Douglassarachne acanthopoda has been described as existing in the famous Mazon Creek locality in Illinois, part of North America’s Carboniferous coal forests.

“This compact arachnid had a body length of about 1.5 centimetres and is characterized by its remarkably robust and spiny legs — such that it is quite unlike any other arachnid known, living or extinct,” said Paul Selden, a researcher at the University of Kansas and the Natural History Museum of London and co-author of the paper presenting the ancient critter.

According to Selden, Carboniferous Coal Measures are an important source of information for fossil arachnids, representing the first time in earth’s history when most living groups of arachnids occurred together. Yet, the fauna was still quite different from today’s.

“Spiders were a rather rare group, only known at that time from primitive lineages, and they shared these ecosystems with various arachnids which have long since died out,” said Jason Dunlop, study co-author and a researcher at the Museum für Naturkunde Berlin. “Douglassarachne acanthopoda is a particularly impressive example of one of these extinct forms. The fossil’s very spiny legs are reminiscent of some modern harvestmen, but its body plan is quite different from a harvestman or any other known arachnid group.”

This led the two scientists to conclude it doesn’t belong in any known arachnid order.

“Unfortunately, details such as the mouth parts cannot be seen, which makes it difficult to say exactly which group of arachnids are its closest relatives,” Selden said.

“It could belong to a wider group, which includes spiders, whip spiders and whip scorpions. Whatever their evolutionary affinities, these spiny arachnids appear to come from a time when arachnids were experimenting with a range of different body plans. Some of these later became extinct, perhaps during the so-called ‘Carboniferous Rainforest Collapse,’ a time shortly after the age of Mazon Creek when the coal forests began to fragment and die off. Or perhaps these strange arachnids clung on until the end of the Permian mass extinction?”

According to the team, the Mazon Creek fossil locality is one of the most important windows into life in the late Carboniferous, producing many fascinating plants and animals. The present fossil was discovered in a clay-ironstone concretion in the 1980s by Bob Masek and later acquired by the David and Sandra Douglass Collection and displayed in their Prehistoric Life Museum.

“The genus name Douglassarachne acknowledges the Douglass family, who kindly donated the specimen to the Field Museum of Natural History in Chicago for scientific study once it became apparent that it represented an undescribed species,” Dunlop said. “Then, acanthopoda refers to the unique and characteristic spiny legs of the animal.”